When a rider enjoys a conventional skateboard, he/she puts his/her one foot on the skateboard provided with wheels, stamps the ground with the other foot in the advancing direction of the skateboard to impart a propulsive force to the skateboard, places his/her both feet on the upper surface of the skateboard in an accelerated state, and alternates the positions of the front and rear feet in directions opposite to each other, so that the skateboard can be moved in a wave-like motion.
A general skateboard is configured to be advanced by the rider's stamping the ground with his/her foot to generate a propulsive force. With this configuration, an improvement in the forward force of the general skateboard is limited, resulting in a reduction of interest in the riding and mobility of the skateboard. Further, rolling casters provided at the lower sides of the skateboard cannot be independently steered, causing a degradation in the ability to steer the skateboard. This degradation poses safety hazards to the rider.
To overcome these problems, skateboards have recently been suggested in which a forward force can be generated by a repeated twisting movement without the rider's stamping the ground with his/her foot.
For example, Korean Patent Application No. 10-2002-25663 suggests a skateboard having a front board 10, a rear board 20 and a connecting element interconnecting the front and rear boards wherein at least one of the boards includes at least one direction-caster 13, 23 attached to the underside of plates 11, 21 and the connecting element includes an elastic member 65 so that the connecting element can be elastically twisted by application of at least one of a twisting or bending force and restores to its original shape when the force is removed.
When a rider alternately rolls the front board 10 and the rear board 20 through a shift of his/her center of gravity and localized distribution of force, the front board 10 and the rear board 20 are elastically rolled due to a restoring force of the elastic member 65, which is included in the connecting element of the front board 10 and the rear board 20 to provide torsional elasticity, so that the boards 10 and 20 are moved forward in a wave-like motion.
Casters 13 and 23 are mounted to the respective front board 10 and the rear board 20 of the skateboard to induce twisting of the front board 10 and the rear board 20 through the connecting element accommodating the elastic member. However, the casters mounted to the respective boards 10 and 20 make it difficult to maintain the balance of the skateboard, thus causing troublesome riding of the skateboard and requiring a long time to learn how to ride the skateboard. For these reasons, the skateboard fails to draw attention and interest of beginners.
Particularly, the elastic member 65 is not an element to directly provide a forward force to the front board 10 and the rear board 20, but indirectly assists in elastically rolling the front board 10 and the rear board 20. That is, since the elastic member 65 is not an element for generating a direct propulsive force to advance the skateboard, the skateboard is not sufficiently moved forward.
In the case where one caster 13 or 23 is mounted to the front board 10 and a pair of casters 13 and 23 are mounted to the rear board 200, safety of the boards 10 and 20 can be ensured but no twisting of the rear board 20 is induced due to a support force of the pair of casters 13 and 23 independently mounted to the rear board 13, causing a problem in that a forward force of the skateboard cannot be generated.
Particularly, when the rear board 20 is biased by excessive twisting during riding of the boards 10 and 20, the caster disposed opposite to the loaded caster of the pair of casters mounted to the rear board 20 is detached from the ground due to a seesaw effect.
That is, when a rider leans the boards 10 and 20 in one direction to advance the boards 10 and 20, his/her center of gravity is concentrated only on the casters 13, which are independently and rotatably installed under the boards 10 and 20, to cause a seesaw effect. The seesaw effect leads to detachment of the oppositely disposed caster 23 from the ground, causing a loss in the balance and equilibrium of the boards 10 and 20. As a result, the boards 10 and 20 are easily overturned, thus posing safety hazards to the rider.
To prevent the overturn of the boards 10 and 20, the rider cannot help riding the skateboard with a large turning radius. This large turning radius lowers the speed and instantaneous steerability of the skateboard.
Further, U.S. Pat. No. 4,082,306 discloses a torsion bar skateboard which comprises a front plate 10, a rear plate 11, a front assembly 12, to which a pair of wheels 17 and 18 are mounted, secured under the front plate 10, and a rear assembly 13, to which another pair of wheels 17 and 18 are mounted, secured under the rear plate 11. The pairs of wheels mounted to the respective assemblies 12 and 13 are secured such that they cannot be rotated for the purpose of steering. A tubing 27 is provided in the skateboard to interconnect the front plate 10 and the rear plate 11 and accommodates an elastic bar 30 therein.
The skateboard is configured to alternately roll the front plate 10 and the rear plate 11 to generate a forward force. The skateboard has a structure in which the assemblies 12 and 13, to which the pairs of wheels 17 and 18 are mounted, are secured under the front plate 10 and the rear plate 11, respectively, so that the pairs of wheels 17 and 18 cannot be voluntarily rotated. Although smooth rolling of the front plate 10 and the rear plate 11 can be achieved through the elastic bar 30 during advance of the skateboard, the wheels 17 and 18 cannot be steered along an advancing direction of the skateboard in a wave-like motion by rolling.
As a result, the steerability of the skateboard is degraded and the rolling forward force of the skateboard is reduced.
As mentioned above, the elastic bar 30 is a constituent element for indirectly supporting the rolling movement of the front plate 10 and the rear plate 11. Since the elastic bar 30 is not an element to directly generate a forward force of the skateboard, the wheels 17 and 18 having no steerability further decrease the forward force of the skateboard and degrade the instantaneous steerability of the skateboard.
Further, U.S. Patent Publication No. 2002-195788 discloses a steerable in-line street ski comprising a plate 11 and a plurality of self-steerable wheels 12 and 13 mounted to both lower sides of the plate 11.
Rotating bodies 15 and 16 capable of rotating at 360° are provided at lower sides of the plate 11 so that the wheels are freely steered along the rolling direction of the plate 1. However, since the front wheel 12 and the rear wheel 13 are integrated with the plate 11, the wheels 12 and 13 are steered in the same direction, depending on the rolling direction of the plate 1. This steering of the wheels 12 and 13 in the same direction weakens the forward force of the in-line street ski and increases the turning radius of the in-line street ski, causing a degradation in instantaneous steerability.
Further, Korean Patent Application No. 10-2005-48075 discloses a roller board which comprises a front board, a rear board disposed at a certain distance apart from the front board, a connecting body for interconnecting the front and rear boards, fixing frames fixed to lower portions of the respective front and rear boards, arms 320a and 320b for supporting respective rollers 330a and 330b, bolts and nuts for rotatably connecting the arms to the frames, and a brake unit for controlling the rotation of the rollers to generate a braking power. The brake unit has an elongated hole formed at the arms, a brake shaft moved up and down along the hole, and a pressing unit for pressurizing the rollers through the brake shaft.
In the roller board, the front and rear boards are connected to the respective rollers 330a and 330b through the arms 320a and 320b rotatably coupled to lower sides of the front and rear board, and the rollers are mounted to the arms 320a and 320b where the rollers are only allowed to be rolled. As a result, the roller board has a considerably large turning radius when cornering.
In case of emergencies during riding, this large turning radius makes quick turning of the roller board difficult without decreasing the speed, resulting in an increased danger of accidents.
Rolling operation is effected by the pair of arms 320a and 320b rotatably coupled to lower sides of the front and rear boards and torsional springs 220a and 222a, and bending operation in left and right directions is effected by a pair of horizontal springs 260a and 260b. A combination of the rolling operation and the bending operation contributes to a forward movement of the roller board. However, since simultaneous realization of the rolling and bending operations in a state in which both feet of a rider are placed on the respective boards is substantially difficult, the rider can hardly keep his/her balance on the roller board, which makes it difficult for a beginner to learn how to ride the roller board.
That is, when bending in left and right directions and rolling are effected to generate a forward force of the roller board, it is difficult for a user to maintain component forces of parallelism in the roller board, resulting in increased occurrence of minor and fatal accidents during riding.